When a pneumatic or hydraulic cylinder is in operation, the piston and piston rod therewithin are of course undergoing rapid acceleration and deceleration as the piston reciprocates. The tool or other load that is attached to the tool plate of the cylinder undergoes the same acceleration and deceleration, as does the fluid within the cylinder as well.
Thus, the moving mass that must be stopped at the end of each piston stroke includes the weight of the piston, the piston rod, the load being moved and the fluid in the cylinder. In an uncushioned cylinder, the kinetic energy of this moving mass is abruptly changed into heat energy as the mass stops at the end of a stroke. In addition to heat, the energy conversion process produces noise and vibration. In a cushioned cylinder, means are provided to gradually decelerate the mass as the piston stroke nears its end, thereby reducing noise and vibration. Perhaps more importantly, a cushioned cylinder requires less maintenance and has a longer working life.
One well known way of cushioning a piston is to mount a cushion spear thereon and to provide a cushion cavity formed in the cylinder head that receives the spear. In a singleacting piston, the spear is mounted on the leading face of the piston in axial alignment with the piston's axis of reciprocation. The cushion cavity is formed in the cap end of the cylinder and is cooperatively aligned. Typically, an adjustable needle valve controls air flow into and out of the cushion cavity to thereby regulate the amount of cushioning provided. For example, with the needle valve wide open, the air in the cavity can escape quickly and the cushioning effect is minimized; conversely, with the needle valve advanced, the air in the cushion cavity is constrained to exit said cavity slowly as the spear enters therein, thereby increasing the cushioning effect. Significantly, such needle valve adjustment is the only heretofore known way to adjust the cushioning effect.
A check valve is also typically provided to allow quick start up when the piston reverses its direction of travel and the spear exits the cushion cavity.
In double acting cylinders, a similar spear, called a spud, is mounted to the opposite face of the piston as well, and a mating cushion cavity is formed in the head end of the cylinder; a similar needle valve and check valve arrangement is also provided at said head end. The head end of the cylinder (the end that receives the piston rod) usually has a larger cushion cavity than the cap end of the cylinder, but the cushioning principle is the same at both ends of the cylinder.
A braking means that floats inside a ram to cushion or brake the end of each stroke of the ram is shown in U.S. Pat. No. 3,824,895, to Martin. A plurality of gauged orifices that progressively control the braking of a piston is shown in U.S. Pat. No. 3,998,132 to Rasigade.
Since the above-described conventional cushioning does reduce noise and vibration and extends the working life of the cushioned cylinder, at least to some extent, most inventors have concluded that the art of cylinder cushioning has completed most of its development, and that only minor refinements remain to be discovered.
However, there art numerous limitations of the known cushioning means. A typical spear or spud extends only three-fourths of an inch from the piston, and the matching cushion cavities are therefore of the same general dimension. Thus, deceleration occurs only during the last three-fourths inch of piston travel. Studies have shown that the known cushioning means cushions only the moving mass of the piston and piston rod itself; the load and the mass of the fluid in the cylinder are essentially uncushioned. Thus, there is a noticeable reduction of noise, vibration and wear in cushioned cylinders when compared with uncushioned cylinders, but the noise, vibration and wear are still substantial.
One obvious way to increase the cushioning effect is to lengthen the axial extent of the spear and spud and hence that of the respective associated cushion cavities, but this solution has never been accepted because it detracts from the length of the piston stroke in a cylinder of the same size or it unduly lengthens the length of the cylinder.
The prior art, taken as a whole, neither teaches nor suggests how this seemingly intractable limitation could be overcome.